I'm no expert, but I have tried some of the methods mentioned above and I find that in the long run I get better results using embossing paste or modeling paste. I buy my modeling paste at a big box store so I can use a coupon.

I found it more difficult to find the same consistency, smooth/creamy feel, with a longer drying time. Embossing/modeling paste doesn't dry as fast so you have time to get the effect you want.

If you aren't sure you're going to like the technique, then make your own in small batches or use spackle, but don't be disappointed if your results are not what you expected. If you need to be frugal, again make your own, but if you can get a 40% off coupon, an 8 oz jar is only about $5.